Cryogenic and membrane synthesis gas production

ABSTRACT

A system and process for the production of synthesis gas with a specific H2CO ratio, for example, 0.7 to 1.2 and a low residual methane content, for example, less than 5 percent (5%). Excess hydrogen from the syngas feed stream is removed by a membrane as a permeate and the retentate gas cryogenically separated.

REFERENCE TO PRIOR APPLICATION

This application incorporates by reference and claims the benefit ofU.S. Provisional Patent Application Ser. No. 60/090,640, filed Jun. 25,1998.

BACKGROUND OF THE INVENTION

Synthetic natural gas or syngas comprises a gaseous mixture derived fromcarbon sources and contains chiefly hydrogen (H₂) and carbon monoxide(CO) together with low amounts of other gases.

Synthetic gases of varying and controlled H₂/CO molar ratios areemployed in various petrochemical processes, such as the production ofmethanol and oxo alcohols, aldehydes, acids, and other chemicalcompounds. One type of synthetic gas is produced by the reaction ofsteam and methane in a reformer, which reaction provides a synthesisgas, after carbon dioxide removal, of about a H₂/CO molar ratio of three(3) and a residual methane content of up to about five percent (5%).

It is desirable to produce a synthesis gas of a selected and lower H₂/COmolar ratio for use in particular petrochemical processes, such as amolar ratio of about 0.9 to 1.0 and also with a lower methane content.

U.S. Pat. No. 5,832,747, issued Nov. 10, 1998, hereby incorporated byreference, discloses a wholly cryogenic process for the cryogenicadjustment of the hydrogen and carbon monoxide molar ratio of a syngas,typically of even molar ratio. The process comprises partiallycondensing, in a cold box, at least a portion of the gaseous feedmixture and then separating the partially condensed feed mixture in aphase separator, to provide a gaseous product stream of a differentH₂/CO molar ratio than the gaseous feed. The process includes warmingthe resultant product stream without any further cryogenic separation.

It is desirable to provide a new and improved process and system forproviding from a syngas, a product gas of a defined selected H₂/CO molarratio and a low methane content.

SUMMARY OF THE INVENTION

The invention comprises a system and process for the production of asynthesis gas having a specific H₂/CO molar ratio, such as in the rangeof 0.7 to 1.2 and a low residual methane content. The system and processcomprises employing a combination of membrane separations to removeexcess hydrogen from the syngas feed, and the subsequent cryogenicseparation of excess methane to provide a gaseous product stream with aselected H₂/CO molar ratio; for example, of about 0.7 to 1.2 or otherselected H₂/CO ratio, and where applicable, a methane content of lessthan about 1.5 percent (1.5%).

In one embodiment, syngas from a steam-methane reformer system with ahigh molar ratio of H₂/CO, and with residual methane, is treated toremove carbon dioxide. The syngas is then heated, typically with steam,and introduced into a semipermeable membrane unit or system. Excesshydrogen is removed as a permeate gas and a retentate or treated gasstream of the desired H₂/CO molar ratio withdrawn; such as, but notlimited to: 0.7 to 1.2, or such other ratio as required in theparticular petrochemical process in which the syngas is to be employed.The excess permeate hydrogen gas may be recompressed and employed forand in subsequent cryogenic cold boxes or employed for other purposes.The retentate gas contains undesirable high amounts of methane, which isremoved by cryogenic fractionation or as a bottoms stream, while thesyngas (H₂/CO) of desired H₂/CO molar ratio is withdrawn as an overheadstream.

The membrane removal of excess hydrogen or other excess gases from thesyngas feed stream is accomplished by one or more semipermeable membranesystems, or units in series or in parallel with the particularsemipermeable membrane material selected for the particular gas to beremoved as an excess gas; such as, but not limited to: polymeric andmetal membranes.

There are a wide variety of membrane materials and designs employed toremove hydrogen gas as a permeate gas. The membrane may comprise hollowfibers or tubes, flat sheets, or spiral wound membranes. Generally, theexcess hydrogen removed as a permeate is at a pressure of about 100 psigand may be recompressed for other uses in the process or in otherprocesses.

The system and process shall be described in a particular embodimentdirected to the treatment of a syngas feed stream from a steam-methanetransformer, with the object to produce a H₂/CO molar ratio of about 0.9and with a low residual methane content of 1.5 percent (1.5%) or lower.However, it is recognized that other syngas treatment streams may beused and other molar ratios obtained by employing a combination of amembrane separations to remove hydrogen from or adjust the feed gasstream composition, followed by cryogenic-fractionating of the retentategas stream, to remove high boiling point materials like alkanes, such asmethane, and to recover an adjusted H₂/CO molar ratio product stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block flow, illustrative diagram of the system and processof the invention; and

FIG. 2 is a schematic process and system illustration of the productionof a synthetic natural gas with a H₂/CO molar ratio of about 0.9 from asteam-methane reformer.

DESCRIPTION OF THE EMBODIMENTS

In FIG. 1, the process steps are numbered by the 10+ series, while thestreams are numbered by the 20+ series.

The hydrocarbon feed 20 may be gaseous, of the form natural gas orliquid, NGL (Natural Gas Liquid) components, ethane or heavier alkanes,or a liquid distillate.

The feedstock, after flowing through the syngas production system 10comprises a mixture of H₂, CO, CO₂ and CH₄. This system may be a steamreformer or a partial oxidation process.

The CO₂ 22 is removed and vented to the atmosphere or recycled to SyngasProduction System 10 to increase the carbon yield of the process.

The raw syngas 21 then enters the membrane system 11, where excesshydrogen is removed as a permeate stream 23. By adjusting thebackpressure on stream 23, the quantity of hydrogen removed can bevaried, and thus, the plant can operate to achieve the desired H₂COratio.

The H₂/CO ratio adjusted syngas 24 now flows to a set of molecular sievebeds 12, where the residual CO₂ and water vapor are removed 25, so thetreated syngas 26 has less than 0.1 ppm of either component to avoidblockages due to formation of solids in the downstream cryogenicprocess.

The cryogenic process 13 employs a distillation column to produce anoverhead vapor product 28, whose methane content is reduced to anadequate level, typically 1½ to 2½ percent for its subsequent usage. Themethane 27 is removed in the column bottoms stream and sent for use asfuel gas or recycled to the reaction step 10.

These process steps: 10, 11, 12, and 13 are operated at the samepressure level in the range of about 200 to 500 psig, typically 300 to400 psig.

FIG. 2 is one embodiment of the cryogenic methane rejection process 13.Process equipment embodiments are referred to by the 100+ series.Process streams embodiments are referred to by the 200+ series.

In the following example of this process configuration, specific processconditions are provided. These process conditions can be adjusted bythose skilled in the art to accommodate changes in feedstock and productrequirements.

The treated syngas 26 comes from the molecular sieve beds 12, where CO₂and water vapor have been removed to low residual values below 0.1 ppm.The conditions of the relevant process streams are listed in thefollowing Table:

Botanical Name Common Name Preferred Source Tanacetum partheniumFeverfew leaf Zingibar officinale Ginger rhizome Curcuma longa Turmericrhizome Coriandrum sativum Cilantro/Coriander seed Centella asiaticaGotu Kola entire plant Oenothera biennis Evening Primrose seed Valerianaofficinalis Valerian root Tabebuia impetiginosa Pau D' Arco bark Thymusvulgaris Thyme leaf Sambucus nigra Elderberry leaf and flower

Feed gas stream 200 enters the multistream heat exchanger 100 throughpassage 100B, where feed gas stream 200 is partially condensed, emergingas stream 201 at −242° F. and 4 percent (4%) molar liquid. This stream201 enters the distillation column 102 operating at 355 psig. Thiscolumn 102 contains trays as vapor/liquid contacting devices 102B shownin dashed lines, and a reflux drum contained by a solid plate 102A atthe top of the column. There is a bottom reboiler 100F and a sidereboiler 100E contained in heat exchanger 100.

The hydrogen/carbon monoxide stream 210 has the methane fractionatedfrom it by the reflux condenser 103. The tower top stream 204 isextracted below plate 102A and enters passage 103A of reflux condenser103, where the tower top stream 204 is partially condensed to stream205, which flows to the reflux drum section above plate 102A containedin column 102. The liquid is separated and returns to the tower asreflux stream 206. The column overhead vapor product is stream 202.

The liquids flowing down the column 102 are stripped of H₂ and CO by thebottom and side reboilers 100E and 100F to produce bottoms stream 203 oflow residual CO content, thereby maximizing the recovery of CO in theoverhead product stream 202. This overhead stream 202 is reheated inexchanger 100 in passage 100A, and exported as the product HYCO(hydrogen/carbon monoxide mix) syngas stream 220.

The column 102 bottoms stream 203 is depressurized to 132 psig, enteringvapor/liquid separation drum 105. Recycle stream 219 combines with itprior to drum 105. The vapor stream 207, at −199° F., is expandedisentropically in expander 104A to stream 210. The liquid stream 208from drum 105 is depressurized to 6.6 psig as steam 209. Streams 209 and210 are combined to form the refrigerating stream 211 in passage 103B inheat exchanger 103. The emerging stream 212 is reheated stream 213 inpassage 100D in exchanger 100.

The reheated stream 213 compressed in the expander 104A drivencompressor 104B from 5 to 10 psig to form compressed stream 214 and thenin subsequent electric motor 106B driven compressor 106A to 145 psig toform stream 215. Aerial aftercooler 107 reduces the temperature of thegas stream 216 to 104° F.

The gas stream 216 is split into methane stream 217 from the cryogenicprocess and recycled stream 218 which is recycled to the cryogenicprocess for the purpose of supplying the refrigeration for the cycle.

The recycled stream 218 is partially condensed in passage 100C ofexchanger 100 to stream 219 at −202° F. and 20 percent (20%) liquid. Thestream 219 enters drum 105 and closes the refrigeration balance for thesystem.

The recycled stream 218 contains CO from the bottoms stream 203 ofcolumn 102 in an amount adequate to provide adequate temperaturedifferential in reflux condenser 103. This CO is also lost in themethane product stream 217.

A method of reducing or avoiding this CO loss is by injecting nitrogeninto the refrigeration cycle, shown and employed to stream 214.Alternative locations where this method may be employed are to streams213 or 218 depending on the pressure of the nitrogen stream. Thisinjected nitrogen is purged from the cycle with the methane stream 217.It cannot contaminate the HYCO product, since the column pressure ofabout 355 psig is higher than the refrigeration cycle pressure of about140 psig.

In this cryogenic process, the distillation column 102 operates at thesyngas feed pressure, the product HYCO syngas stream 220 emerges at thesystem pressure drop lower pressure level. The example is shown with thefeed gas stream 200 at 363 psig. The operating pressure of the systemcan be in the range of 200 to 500 psig.

what is claimed is:
 1. A process for adjusting the molar ratio of H₂/COand reducing the methane level in a syngas feed stream which containshydrogen, carbon monoxide and methane, and which process comprises: a)removing excess hydrogen from the syngas feed stream by a semipermeablemembrane, to provide an excess hydrogen permeate stream and a retentatesyngas stream of selected H₂/CO molar ratio and methane; b)cryogenically separating the methane from the retentate syngas stream;and c) recovering a syngas product stream of selected and lower H₂/COmolar ratio and lower the methane content than the syngas feed stream.2. The process of claim 1 wherein the syngas feed stream comprises a gasfeed stream from a steam-methane reformer.
 3. The process of claim 1wherein the syngas product stream has a H₂/CO molar ratio of about 0.7to 1.2.
 4. The process of claim 1 wherein the syngas product stream hasa methane content of less than about 1.5 percent (1.5%).
 5. The processof claim 1 wherein the syngas feed stream comprises a H₂/CO ratio ofabout 3 and has a methane content of about 3 to 15 percent.
 6. Theprocess of claim 1 which includes compressing the permeate hydrogen andrecovering the compressed hydrogen.
 7. The process of claim 1 whichincludes cryogenically separating the retentate gas stream, by coolingthe retentate gas stream in a multistream heat exchanger, and separatingthe cooled stream in a cryogenic fractionating column into methane as abottoms stream, and the H₂/CO of required molar ratio as an overheadproduct stream.
 8. The process of claim 7 which includes employing themethane bottoms stream as a refrigerant for an overhead condenser to thefractionating column.
 9. The process of claim 8 which includes employingthe methane bottoms stream as a refrigerant in the multistream heatexchanger.
 10. A system for the adjusting of the molar ratio of H₂/CO ina syngas feed stream to a lower H₂/CO ratio and with low methanecontent, which system comprises: a) a steam-methane reformer to providea syngas feed stream; b) a semipermeable membrane system to provide apermeate stream of excess hydrogen and a retentate stream of selectedlower H₂/CO molar ratio; c) a multistream heat exchanger to cool theretentate stream, and a H₂/CO product stream employing a recoveredmethane stream; d) a cryogenic fractionating column to separate a cooledretentate stream into an overhead product stream of a selected and lowerH₂/CO ratio, and a methane bottoms stream for use in the multistreamheat exchanger; e) a means to withdraw a cooled H₂/CO molar ratioproduct stream from the multistream heat exchanger; and f) a means towithdraw a methane stream.
 11. A system for adjusting the H₂/CO molarratio of a syngas feed stream to a lower ratio and less than about 1.5percent (1.5%) methane content, which system comprises: a) asemipermeable membrane means to separate to syngas feed stream into apermeate stream of excess hydrogen and a retentate stream of selectedH₂/CO molar ratio and methane; and b) a cryogenic separating means toseparate the retentate stream into a methane stream and a syngas productstream of a selected and lower H₂/CO molar ratio and less than about 1.5percent (1.5%) methane.